Water cooled nuclear reactors and in particular pressurized water nuclear reactors comprise assemblies formed by a bundle of fuel rods of great length arranged parallel to one another and maintained in a framework formed by guide tubes, spacer elements and two end nozzles. The guide tubes extend in the longitudinal direction of the assembly and are connected to transverse spacer elements evenly spaced apart along the length of the assembly.
The guide tubes are also connected at each of their ends to one or the other of the two end nozzles constituting parts which stiffen and close the assembly.
The fuel rods of the assembly constitute a bundle in which the rods are parallel to one another and disposed in the transverse sections of the assembly in accordance with an even network determined by the spacer elements. Some locations of the network are occupied by guide tubes which are generally rigidly connected to the spacer elements.
The guide tubes have a length exceeding the length of the fuel rods and are placed in the bundle in such manner as to comprise a part which projects from the fuel rod bundle at each of their ends. The end nozzles are fixed to these projecting parts of the guide tubes so as to close the assembly at each of its ends.
The fuel rods are constituted by sintered pellets of nuclear fuel material stacked inside a metal sheath insulating the pellets from the fluid surrounding the fuel assembly.
In the event of a fracture of the sheath of a rod of a fuel assembly, the fuel assembly must be very rapidly replaced in order to avoid leakage of the radioactive product in the reactor coolant fluid.
The fuel assemblies having defective rods are extracted from the vessel of the reactor in which the core is disposed during a shutdown of the reactor for repairs and/or recharging. These assemblies are placed in a storage or deactivation pool placed adjacent the building of the nuclear reactor.
In order to be able to re-use the assemblies having defective fuel rods, it is desirable to be able to replace by new rods the defective rods whose sheath is cracked.
For the purpose of carrying out operations for replacing defective rods in the storage pool, fuel assemblies have been constructed comprising at least one dismountable end nozzle. After dismounting the end nozzle in the storage pool, access may be gained to the fuel rods for replacing the latter. The dismounting of the end nozzle involves the elimination of the connections between the ends of the guide tubes and the end nozzle.
The dismountable end nozzles of fuel assemblies comprise through openings reproducing the network of the guide tubes in each of which a guide tube is engaged and fixed.
The fuel assemblies which permit the replacement of the defective rods or other operations in the network of fuel rods comprise guide tubes of which the connection with at least one of the end nozzles is dismountable.
In order to replace the defective fuel rods, the assembly is placed in a vertical position under water inside a storage cell in the fuel pool.
The storage cell rests on the bottom of the pool and maintains the assembly in the vertical storage position. In this position, the upper end nozzle of the assembly is accessible under a given water head from above the pool, optionally after having slightly raised the assembly inside the cell.
In a known type of a dismountable fuel assembly, the parts of the guide tubes engaged in the upper end nozzle of the assembly comprise a radially expansible part which may be formed by the end part of the guide tube in which longitudinal slots have been machined so as to define flexible cylindrical sectors, or may be formed by an expansible split collar mounted on the end of the guide tube.
The end of the guide tube or the split collar, which may have a slightly tapered shape are adapted to be engaged in a part of the opening of the end nozzle of corresponding shape.
The end part of the guide tube or of the expansible collar comprise a radially projecting part adapted to be engaged in an annular recess machined in the opening of the end nozzle.
The guide tube is fixed inside the end nozzle by a locking sleeve which is inserted in the guide tube and effects the radial expansion of the split collar and the fastening of the guide tube whose radially projecting part engages in the recess machined in the end nozzle.
The locking sleeve generally comprises a locking part proper of generally tapered or frustoconical shape adapted to be engaged in the end part of the guide tube, and a fixing part constituted by a deformable collar which may be set in recesses machined in the opening of the end nozzle outside the zone in which the guide tube is engaged. In this way, the locking sleeve is fixed in position and cannot be lost. The locking sleeves comprise on their inner surface a hooking groove in which may be engaged the claws of a grappling device for extracting the locking sleeve, possibly after disengagement of the set parts of the collar from the recesses of the end nozzle.
Devices for extracting and mounting a locking sleeve of a fuel assembly guide tube are known in the art, for example that disclosed in French patent Patent No. 2,636,766, which comprise a very long pole or boom fixed in a vertical position to handling and hoisting means which are movable above the pool of the reactor. The pole comprises a lower end part which has a diameter less than the inside diameter of the locking sleeve and in which the claws of the hooking grappling device for the sleeve are mounted.
A rod disposed on the axis of the pole and mounted to be movable in the axial direction permits actuating the claws so as to shift them from their retracted position in the lower part of the pole and a projecting position for hooking onto the locking sleeve.
To extract a locking sleeve, the pole is lowered into the storage pool over the fuel assembly in such manner that its lower part can be engaged inside the locking sleeve of a guide tube.
The engagement of the lower end of the pole is facilitated by the fact that the lower end of the pole terminates in the form of an engaging cone. The lower part of the pole is engaged in the sleeve until abutment means mounted on the pole come to bear against a shoulder inside the sleeve of the fuel assembly. The hooking claws of the pole are then at the level of the hooking groove of the locking sleeve. The actuating rod permits shifting the claws from their retracted position inside the lower part of the pole to the projecting position in which the claws enter the hooking groove of the sleeve.
The locking sleeve may then be extracted by pulling on the pole vertically upwardly.
A locking sleeve may be placed in position and mounted in a guide tube of a dismountable end nozzle by means of a mounting device the construction of which is substantially similar to that of the extracting tool.
It is also possible to employ the same tooling for mounting and extracting locking sleeves of dismountable end nozzles of fuel assemblies.
The locking sleeve is mounted and placed in position by inserting the lower part of the pole in a locking sleeve disposed in a store or storage zone in the vicinity of the fuel assembly on which an end nozzle is remounted.
A device for maintaining the sleeve fixed on the outer surface of the pole, or hooking claws placed in confronting relation to the hooking groove of the sleeve, permits picking up the sleeve which may in this way be conveyed by means of the pole to a zone located exactly in vertical alignment with the opening of the end nozzle and with the guide tube engaged in this opening in which the locking sleeve must be placed.
The locking sleeve is then inserted in the guide tube by axially shifting the sleeve.
The sleeve is placed in the locking position generally by means of a thrust device which engages a bearing ledge in the bore of the locking sleeve. In this way the sleeve is mounted and engaged forcefully in its locking position.
The mounting of the sleeve is completed by a setting or punching operation on the deformable parts of the collar constituting the upper part of the locking sleeve to cause them to penetrate the recesses machined in the end nozzle and opening onto the guide tube receiving opening. This operation is generally carried out with a tool which is different from the mounting and extracting tool.
Certain difficulties are encountered, in the use of a device for mounting locking sleeves of the type described hereinbefore, in the case where the upper part of the guide tube is slightly deformed, in particular in the case where some of the cylindrical sectors constituting flexible strips separated by the slots in the end part of the guide tube are bent toward the interior of the bore of the guide tube. The end part of the pole which includes an engagement cone, may be inserted in the bore of the guide tube notwithstanding the reduction of the section due to the deformation of the flexible strips, but the end of the locking sleeve engaged on the lower end part of the pole is liable to assume a position of abutment on the ends of the flexible strips bent inwardly, or even overlap these end parts. In this case, if the imperfect engagement of the sleeve is not immediately detected, the end of the guide tube may be deformed and seriously damaged.
It is then necessary to carry out repairs which may be long and costly.
The operations for mounting the end nozzle of the fuel assembly which depend on the state of the end of the guide tube are therefore not all carried out in a very reliable and rapid manner.